1. Field of the Invention
The present invention relates to a process for producing a polyolefin resin composition containing one or two or more polyolefin ingredients and a filter device for use in this.
2. Related Background Art
Polyolefin resin compositions are excellent in various performances such as mechanical strength, heat seal performance, chemical resistance and food hygienic properties, and therefore they are widely used as materials of polymer films, and so on. In recent years, these polyolefin resin compositions have begun to be used for packaging of more expensive contents and for members of expensive industrial products, and demand for their quality has become severer than before.
If contaminants such as gel-like carbonized polymers or dust are contained in a polyolefin resin composition, a minute defect is caused on the surface of a molded film of the polyolefin resin composition, which results in deterioration in appearance. This circular defect is sometimes called a “fisheye” because of its similarity to the shape of a fisheye.
On this account, as a method for removing contaminants from a polyolefin resin composition, methods in which the composition is filtered through a metal mesh, sintered metal filters such as a metal fiber sintered body and a metallic powder sintered body are known (see, for example, Japanese Patent Laid-Open No. 2006-88081).
A filter excellent in filtration accuracy is usually used in the filtration method mentioned above so as to sufficiently suppress the occurrence of the minute defects (fisheyes). However, when a higher quality is required, use of the filter having excellent filtration accuracy may be insufficient to give a resin composition which meets the requirements. Accordingly, the structure of the filter itself is studied in addition to the improvement in the filtration accuracy of the filter. For example, Japanese Patent Laid-Open No. 2006-88081, Japanese Patent Laid-Open No. H9-38423, and WO 2003/099417 describe filtration method using a leaf disk type filter or a pleated filter.
In the meantime, among polyolefin resin compositions, polyolefin resin compositions containing a plurality of polyolefin ingredients are studied in order to meet the demands for achieving various kinds of physical properties and workability (low probability of causing “die drool” or “streaks”) in the high level with the diversification of products. Since such polyolefin resin compositions are liable to cause gel due to poor dispersion of each ingredient, techniques to use the filter of various kinds have been suggested (see, for example, National Publication of International Patent Application No. 2000-511967).